Interferometers are widely used in science and industry for the measurement of small displacements, surface topography, and changes in index of refraction of an optical medium. Sensors requiring the sensing of small displacements include accelerometers, seismometers, some forms of Light Imaging Detection and Ranging (LiDAR) and displacement sensors for precision machine and lithography tool positioning. Surface topography applications include manufacturing defect detection, 3D scanning, and facial recognition. Sensors that detect changes in the index of refraction of optical medium include some strain sensors and magnetometers. However, interferometers have had limited use in many of these applications.
Interferometers can measure extremely small displacements on the order of an atomic nucleus. But basic interferometers suffer from extremely short operating ranges because of their inherent sinusoidal output that cycles on the order of every optical wavelength in displacement. Devices can be used to increase the linear operating range of interferometers, but these devices have large Size, Weight and Power (SWaP) and are relatively expensive. Present interferometric based sensors have very limited acceptance in many applications because of their high SWaP and cost.
An example of a known interferometer that is configured to measure the displacement of an object is a Michelson interferometer. With a Michelson interferometer, light from a laser is split into two beams, one that is sent along the sensing leg to the object and the other that sent along a reference leg. A retro-reflector on the object reflects the measurement beam back into the opposite direction, where it is reflected again by the beam splitter/combiner and sent to a photodetector. The reference beam is reflected off a mirror, sent to the beam splitter/combiner, where it too is directed to the photodetector to interfere with the sensing beam. Displacement of the object results in a relative phase change between the sensing and reference beams at the photodetector. The relative phase change causes a change in the detected optical power at the photodetector. The detected optical power as a function of displacement of the object is a raised cosine function. For this configuration, the optical intensity completes one full cycle for a half of a wavelength change in displacement. For a semiconductor laser with a wavelength of 1550 nm, the interferometer output completes one full cycle for every 775 nm change in displacement, hence the high resolution of interferometers. However, most applications require a sensor output that is linear over a much larger range than a few nanometers.
One example of a present method for linearizing the sensor output is to use a linear translation stage in the reference arm of the interferometer. A signal at the photodetector output is used in a feedback loop that controls the linear stage to keep the interferometer at some specified lock point. In this case, the output of the sensor is then the displacement of the linear stage. Not only does the linear stage add significant increases in SWaP and cost to the sensor, but also degrades the sensor performance in terms of minimum detectable displacement, or other measurand such as acceleration.
Another prior art method for linearizing the output of an interferometer is to use an optical heterodyne method. In this method, a reference beam is sent through a frequency shifter, which is most commonly an acousto-optic modulator (AOM), before it is interfered with the sensing beam. By frequency shifting the reference beam, the interference between it and the sensing beam will produce a beat note on the photodiode that is at the frequency difference between the two beams. If the object is at rest, the frequency of the beat note will be the same as the frequency of the RF drive to the AOM. If the object is moving, the difference between the frequency of the beat note and the frequency of the AOM RF drive is proportional to the velocity of the object. To obtain the displacement of the object, the frequency difference between the beat note and the AOM RF drive is integrated. The advantage of this method is that it doesn't require any moving parts like the translation stage in the first method. However, the AOM device is relatively large, requires a lot of electrical power and is expensive. Furthermore, since the frequency change of the beat note is integrated to obtain displacement, noise in the frequency measurement will cause random walk in the sensor output, thus eventually resulting in sensor output saturation. Furthermore, offset errors in the frequency measurement will cause the sensor output of indicated displacement to ramp with time. Because the frequency measurement offset may drift with time or environmental changes, the ramp error in the output will have some degree of uncertainty. Noise and offset errors in the frequency measurement precludes the use of this method from many applications.